TESTING THE MICIiOSCOPE 173 



7. Testing the Binocular. — This should be compared 

 with the monocular tube, one eye at a time; a test object 

 being used, and the illumination equalized in both cases. 

 The objective centering may be tested by seeing if the 

 center is changed when shifting from monocular to 

 binocular. 



8. Testing the Photographic Attachment. — Since flatness 

 of field and accuracy of focusing are the main points, a 

 photograph may be taken of the silver hnes of the Abbe 

 test plate, after the correct time of exposure has been 

 ascertained by the Goldberg wedge. The centering should 

 be unaltered, after screwing on the attachment, from what 

 it was before. 



9. Testing the Dark -field Condenser. — Small particles 

 of silver on the Abbe test plate have been used for this. 

 The living spirilla and spirochsetes of the teeth are com- 

 monly employed. Test diatoms give good results on a 

 slide of the right thickness. The star test is usually too 

 delicate for the condenser. 



10. A Regular Test for the High-power Microscope. — 

 The four or five most difficult objects on Moeller's test 

 slide of 20 diatoms afford a good test for regular use; 

 the illumination being constant as to color, intensity, and 

 aperture, and the eye being unfatigued; or a slide of 

 Surirella gemma in hyrax may well be employed; or a 

 well-fixed, well-stained, gentian-violet smear of spiro- 

 chsetes from the mouth. 



11. The Star Test for the Objective.— This method, 

 lately fully described by Coles (47), has several advantages. 

 It consists in focusing the condenser (with a % or %o cone) 

 on a slide having an opaque film of silver (or a nigrosin 

 smear) deposited on it, under a cover-glass of standard 

 thickness, with immersion oil (or balsam) between. If 

 there is undercorrection for axial spherical aberration, 

 too thin a cover-glass, too short tube length, or too high a 

 number on the correction collar, the diffraction rings 

 seen around a minute hole in the silver (or nigrosin) 

 film, are most conspicuous when the focus of the microscope 



